knock knees with biped - problem...

[simesf]

Hello all,

I used biped with default settings to rig a character and while it's generally fine I have a problem whereby if I lift a leg and bend it as part of a walk cycle I get the knees going in towards the centre of the body. Could this be due to me not setting up those 3 little cubes around the heel properly when matching the rig to the model? If not, any other ideas? Also, any idea how I could fix it? Or do I have to go back to the beginning and rerig the character keeping - what? - in mind? Any help would be appreciated.

simesf

update - I went back to the original base before I set up the actual biped rig and enveloped the character. I noticed that the knees were slightly knock kneed there. When I re-enveloped the character the problem was solved. So, would there be a straightforward way to adjust the thigh and shin bone alignment for the original rig or is it time to go back and start again?

[P4wood]

Sorry that I can't help you on this. I'm glad that you managed to find the long solution but it would be handy to know if there is a way to avoid starting from scratch again. I am new to XSI too and am experiencing a similar problem, off and on. Maybe someone can kill two birds with one stone.

I have set up the biped guide several times for this model, and have had different results when the biped rig is generated. The model in the attached image bends his knees backwards when I lower the upper body. I do have a version where the knees bend correctly but haven't got a clue why. I must be placing something in the wrong place in this version but I don't know what it is.

Can anyone suggest what it is in the biped guide that determines the direction that the knee will bend? There are upper knee and lower knee guides but it is not clear where they sould be placed.

[franky]

you need to put at least a small angle when placing the hip. knee, ankle helpers. the knee needs to be a bit off the straight line from hip to ankle in that direction, you wanna bend the knee later.
that gives the IK a clue where to bend it when pieces start moving...

[simesf]

From my very limited experience I suggest that when you are setting up the various cubes and bones that determine where your rig will go that you make sure the bones bend ever so slightly in the direction that you would want them to bend when the character is rigged. & if you don't want a bend at all in, say, the Y axis you wake sure that the two (eg) leg bones form as straight a line as possible with each other. They can be diagonal, but at the same diagonal angle. I hope that's clearer than it sounds in my head...

[P4wood]

Franky and Simesf, thank you both for your quick replies!
Okay, three people have given me much the same advice so I guess I had better set up the guide rig again and take care to add a small bend to the knees.

Much appreciated! :)

[P4wood]

Well, after another attempt I am still experiencing the backwards knee bends.

I setup the guide rig and made certain that the knees had a slight bend to indicate the direction that they would bend. I had XSI generate the rig and then I enveloped the model and adjusted a few of the weights. I then created some simple up/down motion for the hip control which forced the character into a squat and then up enough to lift the heels off the ground plane and then back down enough to force the knees to bend. As you can see in the attached QT file the knees work correctly until the heels leave the ground. It is as if a constraint is being changed when the hip control is raised above a certain hight. I should just mention that once the knees have bent backwards I am unable to make them bend forward again.

I had to dive into XSI quickly and haven't had time to read through all the documentation properly, or in the correct order, so I think I had better try that. I'm sure it will teach me alot and keep me from asking stupid questions. If, however, you can suggest what is likely to be causing the knees to bend backwards I would be very pleased to hear your theories.

Thanks.

Paul :)

[simesf]

It's beyond my reckoning to know what to say - maybe you should consider investing in a character rigging tutorial DVD? Digital Tutors do one, as do 3DQuakers.

[3DMastermind]

What you re experiencing with the knees bending backwards is the same problem as described on this thread:
http://community.softimage.com/showthread.php?t=1327

I wrote a wiki article to fix this problem:
http://softimage.wiki.avid.com/index.php/PRB:_Legs_on_a_Biped_rig_pop_to_the_opposite_side

Also, I highly recommend you check my first training DVD.
It will quickly allow you to understand all aspects of rigging and animation and avoid down time like this.

[P4wood]

Manny, thank you so much for pointing me to your tutorial and your DVD. I will be getting back to this problem soon.
Much appreciated.

Paul
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Edit:
I think that I must have accidentally weighted some points to the bones that you mention in your tutorial. Thanks again.